Purpose: The objectives of this study were to determine Salmonella transfer coefficients (TCs) between (i) new, used and dirty inoculated cartons and tomatoes and (ii) inoculated tomatoes and new, used and dirty cartons, under varying inoculation conditions, contact times and temperatures.
Methods: A five-strain cocktail of rifampicin-resistant Salmonella was spot inoculated onto 5 x 5 cm carton pieces to obtain a final concentration of ca. 6 log CFU/carton. Cartons were either touched immediately to mature green tomatoes (wet) or allowed to dry for 1 or 24 h before contact. Tomato/cartons were subjected to three different contact times (touch (0), 1, and 7 days), at 25°C or 12°C. All sampling was done with 10 replicates. Bacterial populations were enumerated on tryptic soy agar supplemented with rifampicin. The transfer direction was then reversed by inoculating the tomato and contacting the carton.
Results: Salmonella populations decreased following inoculation onto new, used and dirty cartons by 2-3 log units during 24 h drying regardless of storage temperature. In general, the highest transfer rates occurred with wet inoculum, regardless of carton type or temperature storage. The highest TCs was a wet, inoculated tomato stored 7 days at 25°C, TC=14.7. Increasing contact time decreased TCs for new cartons, but increased TCs for used and dirty cartons. A greater percentage of Salmonella transferred from tomatoes to cartons than from cartons to tomatoes, regardless of carton type or storage temperature. For example, TC=0.27 from a wet, new carton, immediately touched to a tomato; while TC=1.38 from the tomato to the carton under the same conditions.
Significance: Salmonella transfer between tomatoes and tomato cartons varies between new, used and dirty tomato cartons, indicating cross-contamination risks may increase under some conditions when cartons are dirty or reused.